This invention relates generally to maintaining a supply of electrical power to electronic components normally furnished from a line supply of electrical power, and particularly relates to maintaining said electrical power to volatile memory components at the occurrence of and during a loss of said line electrical power.
Volatile memory components must receive electrical power continuously to maintain the information stored in their memory cells. Upon loss of electrical power, the states of the memory cells become indeterminate, and any desired information states, such as a logical 1 or a logical 0 at selected addresses, become lost. If the information stored in these volatile memory components is duplicated in non-volatile memory devices, such as a magnetic disk or in a magnetic bubble pack, a loss of electrical power is not catastrophic and is within acceptable operation. Not all central processor systems however use a magnetic disk or bubble memory pack to retain information.
Some volatile memory components such as CMOS (complimentary metal oxide silicon) RAM (random access memory) components operate at extremely low power levels from a 5 volt supply. These CMOS RAM components retain their stored information for extended periods with an extremely low current from the supply; removing the supply current however results in loss of the stored information. A small battery, which supplies about 3.6 volts, carried on a circuit card assembly also carrying the CMOS RAM components can maintain a supply of electrical power during a loss of line source electrical power or during removal of the card from the enclosure of the associated central processor to retain the information in the CMOS RAM components. In this arrangement, the battery is substantially the sole source of back-up power and any by-pass or filter capacitors are intended to source or supply little or no current or voltage.
Other arrangements include a high-capacitance capacitor mounted on the circuit card assembly to maintain some power to the volatile memory components at loss of line source electrical power. In these arrangements, the line source supply charges the capacitor to the nominal 5 volts from the supply and upon failure of the line source, the capacitor supplies power to the memory components. Usually, no battery is present.
Depending on temperature, a battery typically can supply electrical power to the volatile components for several years, while a high capacitance capacitor typically can supply electrical power for many hours.
The switchover from line source electrical power to battery- or capacitor-backed electrical power must be performed cleanly to avoid introducing electrical noise into the electrical power supplied to the volatile memory components. Electrical noise in the supply to the memory components can deleteriously affect the contents of the memory cells to change the stored logical states. Some of these prior arrangements have included standard diodes that exhibit a nominal 0.7 volt forward voltage drop. These
diodes prevent reverse bias of the battery during normal line source operation and conduct electrical power from the battery to the memory components during loss of the line source. The 0.7 volt drop across the standard diode, however, reduces by that amount the typical 3.6 volts of electrical power normally available from the battery for supplying the memory components.
Typically, the central processor or power supply for the central processor can determine the loss of the line source electrical power and provide for an orderly end of the processing of information.
While the use of battery-backed memory card assemblies have furnished a good alternative to non-volatile memory systems, there are some drawbacks associated with them. For example, a battery has a best case shelf life of 6-7 years, which is substantially less than the expected 20-30 years life of the components in associated central processor. This requires that the battery must be replaced at a worst case intervals of every few years to guarantee the integrity of the stored information. Skilled labor must replace the battery and a higher paid manager must keep proper records to insure that the skilled laborer changes the battery at a proper time. These labor costs increase with many battery-backed card assemblies distributed through out a large manufacturing plant. Additionally, special care must be taken to design the central processor cabinet and the circuit card assembly for easy access in changing the battery.
The major event that reduces the battery's shelf life is a frequent and short term supplying of power to the memory components. This often occurs upon momentary failure of line power and upon removing power from the central processor to service a controlled device, to service the central processor and to repair or replace associated equipment. These occurrences usually last from a moment to an hour and have simply been accepted as normal operating conditions that become accounted in determining the replacement interval for the battery.